Find integer that appears exactly once in array of unsorted integers in Java

I have been given an unsorted array of integers where each integer appears precisely twice, excluding one integer which appears only one time. I would like to write a program in Java that finds the integer that appears only once.

Here is my attempt:

int findIntegerThatOccursOnce(int[] arr)
{
    HashSet<Integer> hashSet = new HashSet<Integer>();
    int mSum = 0;
    for(int i = 0; i < arr.length; i++)
    {
        if(hashSet.contains(arr[i]))
        {
          mSum = mSum - arr[i];
        }
        else
        {
          hashSet.add(arr[i]);
          mSum = mSum + arr[i];
        }
    }
    return mSum;
}

My professor said it was a good attempt but there is a better one that uses less space but I cannot see how I can do it with less space? Can anyone help explain about the space issue?